1. Field of the Invention
This invention relates to improvements in a method of thermally decomposing a hydrocarbon, wherein useful olefins such as ethylene and propylene are manufactured by thermally decomposing a hydrocarbon, and in a construction of a thermal decomposition tube.
2. Description of the Related Art
In general, for manufacturing olefin, particularly, ethylene by thermally decomposing hydrocarbons, there has been adopted such a method that a thermal decomposition furnace incorporating therein thermal decomposition tubes is used, a hydrocarbon is heated to a predetermined temperature to be thermally decomposed, and thereafter, quenched. In this case, in order to increase the heat transfer rate to a fluid in the thermal decomposition tubes, such various contrivances have been made such that fins or raised portions are provided on the inner surfaces or the outer surfaces of the thermal decomposition tubes, and the tubes are formed to have elliptical sectional configurations. In order to improve the yield of an olefin, and particularly, the yield of useful ethylene, such contrivances have been made so that the residence time in the thermal decomposition tubes are shortened, and cooled as quick as possible and so on.
However, if the reaction conditions are made too severe to improve the degree of conversion, then the adhesion of the produced coke to the interior of the thermal decomposition tubes becomes notable and the necessity of frequently performing operations of removing the adhering coke occurs. Heretofore, the efficiency of the method of thermal decomposition as a whole has not been improved.
With respect to the thermal decomposition tubes, there has been known that in order to raise the heat transfer rate in the tube, a metal tube provided on the inner surface thereof with spiral fins is useful as a thermal decomposition tube in an apparatus for thermally decomposing hydrocarbons (Japanese Patent Kokai (Laid-Open) Nos. 58-132081 and 58-173022).
However, since the thermal decomposition tube for hydrocarbons is used under particularly severe conditions, the adhesion of the produced coke is heavy, thus presenting such disadvantages that, in order to secure a given operating efficiency, coke must be frequently removed, which is found to be too troublesome.
To manufacture a metal tube having a predetermined length in more then one structure, it is technically difficult from the viewpoint of the manufacturing technology and the like in that a number of metal tubes should necessarily be connected to each other by welding. However, it is difficult to butt-weld these metal tubes with the end faces of the fins perfectly coinciding with each other, thus causing a problem of coke notably adhering to the portions of the fins that do not perfectly coincide with each other, thereby lowering the operating efficiency.
However, heretofore, the fact has been that the shape and the like of the thermal decomposition tube were not studied in detail.